Leo-based positioning system for indoor and stand-alone navigation

ABSTRACT

A method for estimating a precise position of a user device from signals from a low earth orbit (LEO) satellite includes receiving at least one carrier signal at a user device, each carrier signal being transmitted a distinct LEO satellite. The user device processes the carrier signals to obtain a first carrier phase information. The user device recalls an inertial position fix derived at an inertial reference unit. The user device derives a position of the user device based on the inertial position fix and the first carrier phase information.

RELATED APPLICATIONS

This application claims priority from a provisional application Ser. No.60/536,823 filed on Jan. 15, 2004 and entitled, “LEO-BASED POSITIONINGSYSTEM FOR INDOOR AND STAND-ALONE NAVIGATION.” That application isincorporated by this reference.

FIELD OF THE INVENTION

This invention relates generally to systems for navigation and, morespecifically, to using LEO satellites for navigation.

BACKGROUND OF THE INVENTION

In many instances, GPS may not be available to a navigation user. Inhostile conditions, GPS may be jammed or otherwise defeated. In indoorapplication, GPS has insufficient power to penetrate the walls ofbuildings.

GPS now provides at least four ranging sources simultaneously whichenables instantaneous, three-dimensional positioning. However, GPS has alow-power signal that limits operations indoors or in conditions ofheavy jamming. A fundamental advantage of the system described herein isthat it simultaneously addresses the limitations of its predecessors,providing a dynamic, three-dimensional, accurate position fixes—evenindoors or in the presence of jamming.

The Low Earth Orbiting (LEO) satellite constellations, such as Iridium,have been suggested as offering a precise user time standard allowingnavigation without using GPS. Patents have been granted for using thetime standard from the LEO satellites for augmenting the functionalityof the GPS system include such patents as U.S. Pat. RE 37,256, issued toCohen, et al. entitled, “System and Method For Generating PrecisePosition Determinations;” U.S. Pat. No. 5,812,961 issued to Enge, et al.entitled, “Method And Receiver Using A Low Earth Orbiting SatelliteSignal To Augment The Global Positioning System;” U.S. Pat. No.5,944,770 issued to Enge, et al. entitled, “Method And Receiver Using ALow Earth Orbiting Satellite Signal To Augment The Global PositioningSystem;” and U.S. Pat. No. 6,373,432 issued to Rabinowitz, et al.entitiled “System Using LEO Satellites For Centimeter-Level Navigation.These patents still rely, in large part, upon the GPS system.

The performance of MEMS technology is evolving rapidly and can often beoptimized for various applications. MEMS technology has been applied totuning forks in order to produce accelerometer that resolve accelerationto an extent to allow some navigational use. Inertial-grade mechanicalinertial units can also provide a means for inertial navigation. In someapplications alternating between GPS navigation and inertial navigationhas been used for navigation where GPS access is intermittent. Suchsystems rely upon the presence of GPS to initially fix a position forsubsequent inertial navigation.

What is needed is a low-cost, accessible means for precise navigationthat operates independently of GPS.

SUMMARY OF THE INVENTION

The present invention comprises a system for leveraging the relativestrengths of at least two navigation systems, an inertial navigationsystem and a LEO satellite navigation system. The inertial navigationsystem is used to ameliorate the integration load on the LEO satellitenavigation processing system and the inertial system provides a goodapproximation of the changing position of the user device where jammingor an indoor environment blocks the use of the LEO satellite navigation.In turn, the LEO satellite navigation system will provide a preciseposition location to refine the inertial position fix wherever a preciseposition is available.

The presently preferred embodiment includes the ability to further fix aposition with an signal input from a ground-based reference station. Theground-based reference station is advantageously positioned to receivesignals from the LEO satellite through a portion of the earth atmospherethat is similar in propagation properties to the portion of earthatmosphere through which the signal received at the user device haspassed to reach the user device.

In accordance with still further aspects of the invention, a positionderived by a GPS positioning system can be used to refine the positionalfix used for both the LEO-based satellite navigation system and theinertial positioning system. Additionally, a positioning of the userdevice at a precisely known position will appropriately refine theinertial position fix as well as the LEO satellite-derived position.

In accordance with yet another aspect of the invention, theadvantageously rapid changing geometry of the LEO satellites allowsrapid convergence of a positional fix.

In accordance with preferred embodiments of the present invention, a LowEarth Orbiting (LEO) satellite constellation, such as Iridium, is usedas a ranging source operating in conjunction with MEMS inertial sensorsand a precise user time standard to offer an effective and low-costapproach to navigation without using GPS. Applications of the presentinvention include (but are not limited to) ultra-wide-band interferenceprotection, anti jam protection, and enhanced ability to use GPSindoors.

In accordance with further aspects of the invention, MEMS inertialsensors are used to further compact a form factor for the user-device.

In accordance with still further aspects of the invention, inertialnavigation sensors significantly reduce integration time on a signaltransmitting over a special Iridium broadcast channel that has no datamodulation (or with schemes that emulate low modulation rates). Theinertial navigation sensor removes much of the effective bound-onintegration time. Integration times of several seconds would allow anextremely weak Iridium signal (that had originally been boosted 30 to 45dB above GPS in power) to be used deep indoors or in conditions of highjamming.

In accordance with other aspects of the invention, the inventive methodand apparatus reduces multipath. Treating the superposition of severalsub-bands as a CDMA signal rather than a TDMA signal allows thesuperposition to be processed by code correlators inside an Iridiumreceiver to estimate multipath in real time.

In accordance with other aspects of the invention, the inventive methodand apparatus combines its results with those of such other satellitenavigation systems as may be available or intermittently available toestablish fixes to augment positioning solutions. The more rangingsources, the better. Examples of other useable satellites are Globalstarand GPS. In general, with more ranging sources, biases can be estimatedmore quickly, and the dependence on the inertial component can beloosened in proportion to the additional ranging data available.

In accordance with still another aspect of the invention, the inventivemethod and apparatus operate as a backup to GPS in case of jamming. Ifthe user has a clear view of the sky as in JPALS applications, thissystem could provide high-accuracy and integrity positioning on astand-alone basis during times when GPS is not available.

As will be readily appreciated from the foregoing summary, the inventionprovides a method for estimating a precise position of a user devicefrom signals from a low earth orbit (LEO) satellite and includesreceiving at least one carrier signal at a user device, each carriersignal being transmitted a distinct LEO satellite. The user deviceprocesses the carrier signals to obtain a first carrier phaseinformation. The user device recalls an inertial position fix derived atan inertial reference unit. The user device derives a position of theuser device based on the inertial position fix and the first carrierphase information.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred and alternative embodiments of the present invention aredescribed in detail below with reference to the following drawings.

FIG. 1 is a block diagram of an indoor positioning system using LEOsatellites;

FIG. 2 is a diagram of a differential positioning system using LEOsatellites;

FIG. 3 a is a graphic representation of a system covariance from LEO andMEMS sources after a first pass;

FIG. 3 b is a graphic representation of a system covariance from LEO andMEMS sources after subsequent passes;

FIG. 4 is a block diagram of a tightly coupled LEO inertial integrator;and,

FIG. 5 is a flowchart to describe a process for deriving a positionbased upon a LEO signal and an inertial position fix.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to satellite navigation. Many specificdetails of certain embodiments of the invention are set forth in thefollowing description and in FIGS. 1 through 5 to provide a thoroughunderstanding of such embodiments. One skilled in the art, however, willunderstand that the present invention may have additional embodiments,or that the present invention may be practiced without several of thedetails described in the following description.

By way of overview, a method for estimating a precise position of a userdevice from signals from a low earth orbit (LEO) satellite includesreceiving at least one carrier signal at a user device, each carriersignal being transmitted a distinct LEO satellite. The user deviceprocesses the carrier signals to obtain a first carrier phaseinformation. The user device recalls an inertial position fix derived atan inertial reference unit. The user device derives a position of theuser device based on the inertial position fix and the first carrierphase information.

FIG. 1 illustrates a preferred system in which Iridium (or other LEO)satellites 12, 14 are used to provide ranging systems to a user inconjunction with one or more reference stations 16, 18. One of theadvantages of using Iridium is that it is able to produce a signal thatis much stronger than that produced by GPS satellites. Depending oncircumstances, the Iridium satellite can be configured to provide userswith approximately 20dB to 40dB or more received power than GPS.

Positioning using a single ranging source in a three-dimensional dynamicenvironment with Iridium differs significantly from previous positioningsystems in that single ranging sources have been limited totwo-dimensioned resolution on an idealized surface. With the NavyNavigation Satellite System known as TRANSIT, for example, the user wasonly able to make quasi-static, two-dimensional measurements that werelimited in accuracy. Normally, a minimum of four operational TRANSITsatellites were needed to provide the required frequency of precisenavigation fixes.

GPS now provides at least four ranging sources simultaneously in orderto enable instantaneous, three-dimensional positioning. However, GPS hasa low-power signal that limits operations indoors or in conditions ofheavy jamming. A fundamental advantage of the inventive system is thatit simultaneously addresses the limitations of its predecessors,providing a dynamic, three-dimensional, accurate position fixes—evenindoors or in the presence of jamming. Augmented positioning usingIridium should be able to achieve suitable performance limitedprincipally by the effects of ambient multipath.

A ground support infrastructure is present to provide differentialreference measurements. In the presently preferred embodiment, areference station 16 receives signals from satellites 12 and 14 usingreference equipment. Such reference equipment can be functionallyidentical to a user equipment 20 differing only in that the localposition of a receiving antenna is precisely know by survey or otherconventional means including GPS positioning.

Differential reference measurement involves the cooperation of at leasttwo receivers, the reference station 16, and the user equipment 20. Thecooperation of the at least two receivers, relies upon a signal 24received at both the reference station 16 and the user equipment 20 aredegraded by virtually the same errors. The cooperation is possible onearth when the signals pass through virtually the same slice ofatmosphere containing the same obstructions to signals 26. To occur onthe surface of the earth, the user equipment 20 and the referencestation 16, generally, can be separated by fewer than approximately athousand kilometers. Where such geometry is present, the signal 24 thatreach both of the user equipment 20 and the reference station 16 willhave traveled through the same obstacles 26 or will be augmented by thesame pattern of jamming.

The reference station 16 provides real-time measurements of the Iridiumclock. A data message 22, which, in the presently preferred embodiment,is transmitted over Iridium from the reference station 16 to the userreceiver 20, provides a real-time range correction to each measurementto account for both Iridium clock errors and atmospheric effectsincluding obstacles 26 or jamming. Since the reference station 16 has noway of knowing which of the many available satellites the user receiver20 might be using to calculate its position, the reference receiver 16quickly runs through all the visible satellites, such as satellite 14,and then computes the error attendant to its signal 28. The correctionsnecessary to bring the calculated result into line with the known localposition of the reference station are then transmitted on any suitableband with adequate confidence in the jamming environment to the userequipment in association with time references to establish near realtime correction. In general, navigation performance degrades asseparation between user and reference station gets greater due toattendant differences in obstacles 26 or jamming the signal 24experiences.

Where a second reference station 18 is suitably close, the secondreference station 18 can perform the same calculations on the signal 28as the first reference station 16 yielding a second correction factorfrom, for instance, the satellite 14, allowing the user equipment toachieve greater precision by averaging or other suitable means ofharmonizing the error calculation.

Referring to FIG. 2, a block diagram for presently preferred systemarchitecture for a positioning system 30 uses Iridium or other LEOsatellites. Each component of the positioning system 30 is driven fromthe same master clock—a precise time standard 40. A synthesizer 38creates each of the requisite coherent sine wave and clock signals foreach component based upon a clock signal fed to the synthesizer 38 fromthe precise time standard 40 through a data bus 42.

An antenna 32 is configured to receive transmissions from the Iridium orother LEO satellites as the presently preferred embodiment is configuredand is optimized for L band reception. An Iridium receiver 34 receives araw signal received at the antenna 32 and compares it with the signalgenerated by the synthesizer 38 and presented to the receiver 34 at adata bus 48. By comparing the signal at the data bus 48 with thetransmission received at the antenna 32, the Iridium receiver 34presents data sufficient to compute a position solution.

An augmented position solution is calculated using an inertialmeasurement unit 36 receiving a clock signal from the synthesizer 38.Measuring acceleration with the inertial measurement unit 36 in thepresently preferred embodiment is accomplished by accelerometersoriented in three orthogonal axes and measuring angular rate about eachsuch axis to compute attitude accurately relative to a vertical axisaccomplish accurate attitude sensing. Attitude and other parameters ororientation and motion of the user are derived from the data produced bythe accelerometers and rate sensors within the common assembly. In thepresently preferred embodiment, the accelerometers are MEMS inertialsensors.

Measuring acceleration with the inertial measurement unit 36 in thepresently preferred embodiment augments the system to provide a systemthat anticipates the next position of the user. Optionally, the positionsolutions derived by use of the inertial measurement unit 36 may beharmonized with earlier solutions to gain a self-testing ability and toreduce a radius of error in the calculation of the position with theinertial measurement unit 36.

Three-dimensional positioning and filtering using Iridium operates overtime scales of about 10 minutes—much less than the 84-minute Schulerperiod. The Schuler period is the period for a simple undamped pendulumwith a length equivalent to the radius of the earth and has been used tocorrect traditional inertial navigation equipment for the curvedmovement of a spot on the surface of the earth. Therefore, the inertialunit needs to be capable of providing relative position measurementswhose accuracy is significantly better than the filtered rangemeasurement accuracy of the Iridium signal.

With MEMS inertial sensors of sufficient performance, degradation due tothe ambient multipath of an indoor environment will dominate the overallsystem-level accuracy. The total system accuracy will start out in the4-meter range representing one sigma in standard deviation. Advancedsignal processing techniques applied to the Iridium signal significantlyreduce indoor multipath error. In outdoor applications with anunobstructed view of the sky, the accuracy will be considerablybetter—limited mostly by the performance of the inertial reference unit.

In a presently preferred embodiment of the invention, the inventivemethod and apparatus creates a Secure Iridium Broadcast Signal. Althoughthe Iridium signal is technically a TDMA signal, the superposition ofseveral sub-bands together to formulate a high-powered signal to appearmore like a secure CDMA signal. With such a formulation, a navigationuser knows the code in advance to be able to make use of it. If thepulse patterns that make up the secure Iridium Broadcast Signal areprogrammed correctly, the high-power signal would appear like the secureY-code signal of GPS or its equivalent for processing.

The systems architecture for the indoor case driven by multipath imposesan implicit requirement on the total position bias of about 1 meterafter 10 minutes of coasting. The limiting inertial parameter is likelyto come from the gyro-rate bias stability or angle random-walk error. Ahigher performance inertial system is required if the system is to beused outdoors for high-accuracy and integrity navigation.

The computer 54 serves to tie all the Iridium ranging measurementstogether especially when there is only a single ranging source in viewat any given time. “High accuracy” means position errors at thecentimeter level. “High integrity” is a safety related term that meansthat there is enough redundant information present in the form of excesssatellite ranging measurements to determine if there is an error in thepositioning system. Such capability can be used to alert an operator ofthe system when that system should not be used for navigation. Highperformance navigation employs the carrier phase of the LEO satellite toattain raw range measurements precise to the centimeter level.

Because the system will often be measuring only one ranging source at atime, it is desirable that a precise frequency standard be used. Twotypes of frequency standards are available for this purpose: an ovenizedquartz crystal oscillator and an atomic rubidium frequency standard. Anovenized quartz crystal as long as the Allan variance at 600 seconds (10minutes) does not exceed 10⁻¹¹. This corresponds to about 2 meter ofposition error over the Iridium pass—significantly less than themultipath error on the Iridium signal. If additional accuracy is needed,a compact, ruggedized rubidium standard should be used. Thecorresponding Allan variance is 10^(—13), corresponding to a positionerror of about 2 cm over the 10-minute interval.

Raw position solutions from the Iridium receiver 34 through a data bus50 and acceleration measurements from the inertial measurement unit 36through a data bus 52 are fed into a computer 54 which executes a Kalmanfilter to process the measurements into final solutions. The Kalmanfilter is a set of mathematical equations that provides an efficientcomputational (recursive) solution of the least-squares method. Thefilter is very powerful in several aspects: it supports estimations ofpast, present, and even future states, and it can do so even when theprecise nature of the modeled system is unknown.

The Kalman filter estimates a process by using a form of feedbackcontrol: the filter estimates the process state at some time and thenobtains feedback in the form of (noisy) measurements. As such, theequations for the Kalman filter fall into two groups: time updateequations and measurement update equations. The time update equationsare responsible for projecting forward (in time) the current state anderror covariance estimates to obtain the a priori estimates for the nexttime step. The measurement update equations are responsible for thefeedback—i.e. for incorporating a new measurement into the a prioriestimate to obtain an improved a posteriori estimate.

Since raw position solutions from the Iridium receiver 34 through a databus 50 and acceleration measurements from the inertial measurement unit36 through a data bus 52 that are fed into the computer 54 aremeasurements of the same phenomenon, i.e. movement in space, themeasurements are related in the system modeled by the Kalman filter 157(FIG. 4).

Depending on the circumstances, not all states (such as yaw attitude)will necessarily be observable at all times. However, because of theorbit geometry of Iridium, the system design ensures that the positioncomponent of output will effectively always be observable to within theaccuracy of the Iridium ranging measurements.

There are two fundamental modes of operation of this invention. Thefirst is based on code phase measurements. Inside of a building, thereare many sources of multipath, so using the carrier is not especiallyfeasible. However, LEO satellites provide an abundance of geometry, asshown in FIG. 3, along with significantly higher broadcast power that isuseful for penetrating physical barriers. The code ranging measurementscan be combined using this geometry to solve for reasonably accurateposition; using the inertial navigation unit to bridge measurements madeat different times.

The second mode of operation is based on carrier phase measurements. Ifcarrier phase measurements are made outdoors, it is possible to obtain aclean line of sight to the LEO satellites, and therefore, achievecentimeter-level positioning accuracy. The same abundance of geometry,as shown in FIG. 3, enables these precision measurements to be combinedinto high accuracy and high integrity position solutions, again usingthe inertial navigation unit to bridge measurements made at differenttimes.

FIG. 3 shows a typical geometry pass from the standpoint of the user.The Iridium satellites fly in an arc over an interval of severalminutes. Multipath will generally be the largest error source. TheIridium carrier phase can be used to drive the ranging error to bearbitrarily small—potentially to centimeter level—when the user has aclear view of the sky. Unfortunately, raw ranging errors will tend toincrease to roughly 20-30 m working indoors. Because the Iridiumsatellite spans a large-angle arc in the sky, it should be possible totake advantage of spatial diversity to average down much of this indoormultipath error. By analogy with experimental GPS performance, it ispossible to predict what Iridium performance is likely to be by scalingthe parameters. The correlation time between Iridium measurements isestimated to be about 10 seconds, meaning that over a 10-minute pass,the receiver can gather roughly 60 “independent” measurements.Therefore, the ranging accuracy may perhaps be improved to roughly 4meters (dividing the raw ranging accuracy by the square root of 60).

As shown in FIG. 3, a cold start initialization 60 uses a trajectory ofthe first Iridium satellite pass to define a local section of theIridium orbit sphere 64 having a zenith 62 relative to the position ofthe user 68. Inertial navigation by the inertial measurement unit 36yields a positional covariance after the first pass 66 as shown relativethe position of the user 68. The rapidly changing angle of the orbit ofthe LEO satellite in the LEO satellite orbit sphere 64 allows for arapid convergence of the position estimate allowable by means of the LEOsatellite in its orbit sphere 64.

The system structure resembles a tightly coupled GPS-Inertial unit.However, as shown in FIG. 4, the system 100 is intended to process asfew as a single range measurement at a time using a Kalman filter 150.For dynamic applications, a MEMS Inertial Reference Unit (IRU) 102 iscoupled to the system and subjected to error preprocessing in the errorpreprocessing unit 105. In more demanding applications, aninertial-grade IRU may be desirable.

A general model for a suitable IRU 102 includes a strapdown inertialnavigation system. Strapdown inertial navigation systems are rigidlyfixed to the moving body. Therefore, strapdown inertial reference unitsmove with the body, their gyros experiencing and measuring the samechanges in angular rate as the body in motion. The strapdown inertialreference unit contains accelerometers to measure changes in linear ratein terms of the body's fixed axes. The body's fixed axes serve as amoving frame of reference as opposed to the constant inertial frame ofreference. The navigation computer uses the gyros' angular informationand the accelerometers' linear information to calculate the body's 3Dmotion with respect to an inertial frame of reference.

The IRU 102 senses inertial acceleration which it outputs as rotationalacceleration. The rotational acceleration vector information is fed intoan error preprocessor 105. The inertial error preprocessor 105 correctspre-calibrated parameters, including scale factor and alignment errors.Next, the corrected measurements pass through the time update blocks 108and 111, including the addition of the accelerometer and gyro biasstates and the integration of the strapdown IRU 102 measurements intoposition, velocity, and attitude vectors.

At an in-phase coordinate processor 114 and a quatrature coordinateprocessor 117, a vector translation, x_(l), and attitude motion,represented by the 3×3 attitude rotation matrix A, of the user platform.With prior knowledge of the antenna mounting lever arm b 120, withrespect to the body frame of the user platform, it is possible to usethe inertial signal output from the in-phase coordinate processor 114and the quatrature coordinate processor 117 to project the antennamotion into the line of sight of the satellite, ŝ at a processor 126.The output of the processor 126 is a complex, real-time phasecorrection. The phase correction is to subtract out short-term usermotion and enable long integration times on a LEO signal, when such aLEO signal is available.

On the LEO receiver side of a LEO (in the case of the presentlypreferred embodiment, an IRIDIUM) receiver 132 receives a carrier signalfrom the LEO satellite. In an presently preferred embodiment, a secondcarrier signal received at a reference ground-station in proximity tothe user device is also received in association with the preciseposition of the ground-station position at an optional datalink 135. Thesecond carrier signal insures a rapid integration of the carrier signalfrom the LEO satellite and further enables operation of an LEO errorpreprocessor 138.

As with the inertial side, the LEO error preprocessor 138 correctspre-calibrated parameters, including scale factor and alignment errors.Additionally, the LEO error preprocessor 138 corrects propagationinduces errors based upon the information received at the optionaldatalink 135. The error processor 138 applies corrections such as foratmospheric/ionospheric effects, time tag alignments, and blending codeand carrier.

Bias state time update blocks 141, 144, 147, and 151 apply the scalarreceiver clock and clock bias estimates to the raw measurements. Afurther bias block 154, uses the output of the processor 126 to subtractout short-term user motion and enable long integration times on a LEOsignal, when such a LEO signal is available. The corrected LEO positionis ready for feeding into the Kalman filter 157. In the presentlypreferred embodiment the computer 54 executes a 17-state Kalman filterestimator to solve for:

Position (3 axes)

Velocity (3 axes)

Accelerometer bias (3 axes)

Attitude (3 axes)

Gyro bias (3 axes)

Clock bias

Clock drift

A covariance time updater 160 propagates a state covariance estimate.The estimated inertial position, projected into the line of sight ofeach given LEO satellite by the processor 126, is compared with themeasured range to the LEO satellite to form the measurement update tothe Kalman filter 157.

Referring to FIG. 5, a method 200 is provided for estimating a preciseposition of a user device in a satellite-based navigation system. At ablock 201, a user device receives transmitted carrier signals from a setof LEO satellites. At a block 204, the user device processed the carriersignals to obtain user carrier phase information including geometricallydiverse user carrier phase information from the set of LEO satellites.At a block 207, the user device recalls an inertial position fix. At ablock 210, the precise position of the user device based on the inertialposition fix and the user carrier phase information. At a block 213, theuser device derives user carrier information from the set of LEOsatellites based upon the inertial position to resolve integer cycleambiguities in the user carrier phase information.

In a preferred embodiment, the method 200 includes tracking the carriersignals at a reference station to, obtain reference carrier phaseinformation. The reference carrier phase information includesgeometrically diverse reference carrier phase information from the setof LEO satellites. At a block 216, the user device refines the accuracyof the position calculation based upon the reference carrier phaseinformation. In a preferred embodiment, the method further comprisesestimating an approximate user position and clock offset using codephase signals received from a set of navigational satellites.

Preferably, differential code phase techniques are used to improve theaccuracy of the initial estimate. The preferred embodiment of the methodalso includes additional advantageous techniques such as: compensatingfor frequency dependent phase delay differences between carrier signalsin user and reference receiver circuits, reading navigation carrierinformation and LEO carrier information within a predetermined timeinterval selected in dependence upon an expected motion of the userreceiver and the LEO signal sources, calibrating LEO oscillatorinstabilities using navigation satellite information, compensating forphase disturbances resulting from a bent pipe LEO communicationarchitecture, compensating for oscillator instabilities in the user andreference receivers, predicting present reference carrier phaseinformation based on past reference carrier phase information, andmonitoring the integrity of the position calculation.

Depending on the circumstances, not all states (such as yaw attitude)will necessarily be observable at all times. Because of the orbitgeometry of Iridium—specifically the rapid large-angle overheadmotion—the system ensures that, upon convergence, the position componentof output will effectively always be observable to within the accuracyof the Iridium ranging measurements.

If high-performance carrier ranging is to be carried out, an optionalfloat bias state is added, one for each LEO satellite, as shown in FIG.4, to account for the integer cycle ambiguity. Accordingly, the scope ofthe invention is not limited by the disclosure of the preferredembodiment. Instead, the invention should be determined entirely byreference to the claims that follow.

While preferred and alternate embodiments of the invention have beenillustrated and described, as noted above, many changes can be madewithout departing from the spirit and scope of the invention.Accordingly, the scope of the invention is not limited by the disclosureof these preferred and alternate embodiments. Instead, the inventionshould be determined entirely by reference to the claims that follow.

1. (canceled)
 2. The method of claim 33, wherein the positional fix isused to refine carrier phase information to resolve integer cycleambiguities
 3. (canceled)
 4. (canceled)
 5. The method of claim 33,wherein the positional fix is used to refine code phase information. 6.(canceled)
 7. The method of claim 33, further comprising deriving thepositional fix from an inertial reference unit.
 8. (canceled)
 9. A userdevice for estimating a precise position from signals from a low earthorbit (LEO) satellite, the user device comprising: a receiver configuredto receive at least one carrier signal at the user device, each carriersignal being transmitted by a distinct LEO satellite; a signal processorconfigured for processing the carrier signals to obtain carrier or codephase information; a positional sensor for recalling an inertialposition fix of the user device; and a processor for determining aprecise position of the user device, the processor using the positionalinformation from the position sensor to bridge the carrier or code phaseinformation.
 10. The user device of claim 9, wherein the processor isfurther configured to resolve integer cycle ambiguities based upon theinertial position.
 11. The user device of claim 9, wherein the receiveris further configured to process the carrier signals received at areference station to obtain a second reference carrier phaseinformation.
 12. The user device of claim 11, wherein the processor isfurther configured to refine the position of the user device based uponthe second reference carrier phase information.
 13. The user device ofclaim 9, wherein the processor is further configured for estimating anapproximate user position and clock offset using code phase signalsreceived from a set of navigational satellites.
 14. The user device ofclaim 9, wherein the position sensor includes an inertial referenceunit.
 15. The user device of claim 14, wherein the inertial referenceunit is a MEMS inertial reference unit.
 16. (canceled)
 17. A computersoftware product comprising a computer readable medium encoded with datafor causing a user device to estimate a precise position of the userdevice, the estimating including: processing carrier signals received bythe user device from at least one low earth orbit (LEO) satellite toobtain carrier or code phase information; recalling an inertial positionfix of the user device; and deriving a precise position of the userdevice based on the inertial position fix and the carrier or code phaseinformation, wherein the inertial position fix is used to bridge thecarrier or code phase information.
 18. The software product of claim 17,wherein carrier phase information is refined to resolve integer cycleambiguities.
 19. The software product of claim 17, wherein the carriersignals received at a reference station are processed to obtain a secondreference carrier phase information.
 20. The software product of claim19, wherein the position of the user device is further refined from thesecond reference carrier phase information.
 21. The software productclaim 17, wherein the position is refined using code phase signals.22-32. (canceled)
 33. A method comprising estimating a precise positionof a user device, including: deriving a positional fix from a positionsensor carried by the user device; receiving signals from a low earthorbit (LEO) satellite and processing those signals to obtain carrier orcode phase information from LEO satellite signals; and using thepositional fix to bridge the carrier or code phase measurements todetermine the position of the user device.